There are two resources which are my personal favorites. First
is the NIGTC commentary on Galatians. The second, which is not
a commentary, but equally important, are two articles in the
1994? Tyndale Bulletin which do an analysis of Paul's rhetorical
structure in Galatians in coversation with both Chrysostom
and Betz (can you get much farther apart theologically?). An
overall understanding of rhetorical practices is essential for
understanding Galatians and I would expect any serious commentary
that comes out these days to deal with it. I've not seen it,
but based on the quality of his other work, I'd expect
Longenecker's WBC volume to be helpful as well.